This invention relates in general to mechanical differentials and more particularly to a pinion shaft within a vehicular differential having a thin film coating that provides a highly wear-resistant and low friction sliding surface, and processes for manufacturing the same. The coating generally comprises a metal- or silicon-containing carbon material that may be applied using various vapor deposition techniques.
Conventionally, when a low viscosity lubricant is used in the differential, high-grade carbon-Mo steel (e.g. JIS SCM 30-40) is used for the pinion shaft, and the shaft is carburized/quenched and soft-nitrided to provide a hardened surface to resist wear. Alternatively, the steel shaft is molybdenum flame-coated or plasma sprayed. However, while abrasive wear-resistance and surface durability is improved by surface hardening, such as soft-nitriding of the higher grade steel, seizure or adhesive wear resistance is not improved that much. On the other hand, although seizure-resistance is improved by molybdenum flame coating, abrasive wear resistance and surface durability are little improved.
Conventional ceramic coatings applied at a temperature lower than the normalizing temperature or surface-treatment temperature, such as of soft-nitriding, gas-nitriding, etc. have been used to improve the pinion shaft wear characteristics. These techniques are generally applied using various methods, such as reactive ion plating, plasma chemical vapor deposition or laser chemical vapor deposition, each applied at a treatment temperature of approximately 500 deg C.
The thickness of these conventional ceramic films is typically 1-20 microns, and preferably 2-10 microns. If it is thinner, improvement of seizure/wear resistance is insufficient and if it is thicker than preferred, the cost of the treatment increases too much. The ceramic used in such conventional techniques can vary widely, and may include materials such as TiN, Ti(C, N), Si3N4, TiC, SiC, Al2O3, etc. It is generally recognized that TiN, Ti (C, N), and Si3N4 are more effective for seizure resistance improvement and TiC, SiC, and Al2O3 are more effective for improvement of wear-resistance.
The base material of the pinion shaft is likewise variable, and can be composed of metals such as SAE/AISI steel types 1045 to 1080, 8620, and 4140 J for example. The heat treatment can be quenching/tempering, high-frequency quenching/tempering, carburizing/tempering or carburizing/nitriding/tempering etc. and soft-nitriding (salt bath or gas) or gas-nitriding can be applied after said heat treatment.
HV 400 or higher surface hardness of the base material underneath the coating is necessary. If softer, the contact stress may be sufficient to cause plastic deformation or cracking of the base material that would lead to the coating becoming detached from the base material. Further, the surface of the base material should be free of any brittle layer, such as a porous compound layer caused by nitriding. If a brittle layer is formed during processing, the ceramic coating must be applied after removal of the brittle layer. These considerations cause unacceptable costs or limit reliability and reduce the life of the differential.
Application of a phosphate treatment (Lubrite treatment) of the pinion is another conventional technique used to reduce the wear between the pinion shaft and pinion.
While all of these conventional techniques provide some improvement to wear of the pinion shaft, their improvements are limited and/or impose unnecessary or excessive costs. It is therefore desirable to have a pinion shaft with greater wear resistance without excessive additional cost.